While the term "wavelength resolution" as applied to a spectrophotometer properly means the smallest increment of wavelength that can be distinguished by the photometric detection system, the term is also commonly used in the field as a synonym for the half bandwidth, often just called "bandwidth", of the instrument. In this disclosure the latter usage will be understood as synonymous with "resolution".
As is well known in the art, wavelength resolution, hereinafter simply called "resolution", is a function of the spectrophotometer determined by the parameters of the monochromator such as physical dimensions, number of lines/mm of the grating, and width of the entrance and exit slits. Other than for multigrating spectrophotometers where gratings of different grooves per millimeter may be interchanged, the resolution can normally be varied only by changing the slit width. Variable width slits, however, are expensive and liable to reproducibility errors since the mechanism required for such a slit must be extremely precise to establish widths of a few micrometers reproducibly. In the case of a monochromator where the photometric device is a photodiode array which takes the place of the exit slit the width of an individual element, or "pixel", of the array may typically be 25 .mu.m. Since each pixel functions as an exit slit for a specific wavelength band the slit width will be normally 25 .mu.m for both exit and entrance slits.
In this invention disclosure the spectrophotometer cited as an example is a scanning spectrophotometer using a photodiode array (PDA) as a photometric detector. While this invention is well suited for providing resolution conversion in conjunction with such a spectrophotometer, it should be also made clear that the invention, with suitable modification, can also be used to advantage with other types of spectrophotometers having fixed entrance and exit slits operated in scanning modes.
It is an object of this invention to provide computer programmed means for establishing a multiplicity of operating bandwidths (resolutions) for an associated spectrophotometer.
It is an object of this invention to be able to operate a spectrophotometer at a plurality of selected resolutions without the use of variable slits.
It is also an object of this invention to be able to use a photodiode array as a photometric receiver in a spectrophotometer while retaining the ability to operate at any of several resolution values.
It is a further object of this invention that digitally computed data output values shall, irrespective of the resolution selected, substantially agree with such values obtainable by physical change of slit width.
It is yet a further object of this invention that the resolution conversion program may be run on an auxiliary data handling computer peripheral to the spectrophotometer.